Anyone with a Custom Rear Subframe?

65Fastback434 · 11-07-2009, 06:30 PM

I know a few people get rid of the unibody and create a whole frame.

Well the front of my car was in pretty decent shape and I repaired all of the rust up there. However the rear is a different story.

The rear subframes are mostly rusted and the PO put some other metal to cover it. The whole trunk floor is rusted out too. And I would like to widen the space in the rear from some nice large tires. I already have a 3-Link sitting in my garage in boxes that wouldn't be too hard to modify to fit.

This is what I was thinking...

These are mandrel bent 2X3x11ga boxed tube that are bent the specs of a 64-70 mustang, the only difference is that they run parallel and don't flare out like factory

I would build this off the car and use a full trunk floor replacement and the parts for my 3 link to mock everything up, and if possible put it all in as one piece. The issue i see myself running into is how to easily keep the rear torque boxes in tact but still be able to remove the rear subframes and attach the new ones

stangg · 11-07-2009, 08:03 PM

I made a removable rear subframe out of 2x3 tubing... straight stock, angled cuts and welded joints. In my case I installed inner rockers for convertible and built my own torque boxes. For the rear section I used the 19" long frame rail patches and inserted 1.5 x 2" tubing that serves as a mounting points for the rear portion of the subframe.

I doubt those back half type rails will conform to the original floor pan, it doesn't look to me like it kicks up enough. As far as installation / mockup I think you would be better off gutting the rusty sheetmetal, fit up the new rails and 3 link, then just buy the seperate components and weld them in one by one.

That one peice floor appears to have the rear cross member in place. I'm also guessing with the 3 link, that you are going to go with a panard bar or watts linkage, and probablt coil overs too. With all that going on, I think you would be better of fitting it up in the car and not off.

Is your three link the lower long torque bar setup or a single short upper link that mounts above the axle? If the latter, you're probably going to need to skip out on the shock mounting cross member and likely run a piece of tubing between the frame rails to support the upper link.

If you've got a fold down seat, be sure to find a way to insure you keep your floor mounted seat brackets in the correct place... but depending on how far in you want to go you may not be able to use the rear seat at all. ( I think about 1" per side is the max).

Something else I've heard is that the 67/8 outer wheel houses are wider than the 65/6 wheel house, and using those pushes the quarter panel out another a bit without being obvious. I'm not sure how much you gain, but it might be another inch per side.
Sort of a free flair if you have to replace the wheel house and quarter panel.

65Fastback434 · 11-07-2009, 08:49 PM

Yeah I have a watts link with the 3 link and it has the long lower torque bar. I too have convertible inner rockers but when I installed them, instead of cutting out the rear torque boxes and having the inner rockers go all the way to the back, I cut the ends of the inner rockers to match the shape of the rear torque boxes and welded them together.

Yeah I've heard about using the 67/8 wells to get an inch or so out of it, but I want a bit more, I would like to fit some 12" wide wheels under there, with about at 14" wide tire, or at least as close to is as possible, so I was thinking of tubbing it and making the tubs a bit wider to force out the quarter panel similar to what the 67/8 would do.

So your saying I should get the boxed frame rails i pictured above installed first then mock up the 3 link, and then place in pieces of the floor? As I said before the trunk and rear frames rails are pretty much dead including the rear truck crossmembers and both quarters have some damage, and have been replaced some time ago by a previous owner, and they did a really bad job, there was a lot of bondo covering the seams, so the whole rear 1/4 of the car needs to pretty much be re-done, and I figure why just repair when I can make it better, and stronger, I want to be able to run a lot of power though the car without the frame getting bent out of shape, so i also plan on installing a pretty substantial roll cage, and do some front suspension triangulation that they did with Julian's Fire and Ice vehicles which I think you are familiar with

stangg · 11-07-2009, 10:07 PM

Yeah if it were my car, I think I’d replace the rear cross member first, and then fit up the rear frame rails ( tack ‘em in ) Figure out how to best tie the front rails into the ‘vert inner rockers. Once the rails are set, set the rear end in place, set the 3 links, set the ride height, set up the watts, then figure out the rear shock position… Once you get your frame, brackets and roll bar points established then weld all the structural things up and then start on the sheet metal work. For some of the sheetmetal you’ll probably be able to rework stock style parts, but I’m thinking the transition panel will have to be custom made to conform to the frame rails and widened inner wheel houses. The trunk drop off panels will need to be reworked. You’ll also probably have to stiffen up the area where the bumper mount brakets mount since the frame rails will likely be further inboard. Be sure to figure out how you’ll mount your fuel tank too.
With everything you have going on, a weld in solution is probably better than trying to figure out how to make it a bolt in assembly. Are you planning to jig up the rear before you get started so that you have a consistent frame of reference for all the new stuff?

rooeng · 11-07-2009, 10:30 PM

I have something similar to what Stangg described that was done by the PO. I've been trying to fix and modify some of the things that were done.

Make sure there's enough room between the frame rails for the mufflers, driveshaft (if you use a 3.5" dia, you'll need even more room), torque arm and the arms if you mount them inside the rails. If you mount them outside, leave enough room for the bolt heads.

If the frame rails too close together, the gas tank won't fit. You'll need enough room between the diff and the tank for the watts, coilovers and exhaust.

You could rebuild it back to original. Use the 67/68 wells for an extra inch plus eliminating the leaf springs and mount the arms on the inside of the frame rails will get you about 3". That'll let you go from a 8" wheel to 12".

BTW it's a Torque Arm Suspension not a 3 link.

65Fastback434 · 11-19-2009, 11:08 AM

Quote:

Originally Posted by stangg

Are you planning to jig up the rear before you get started so that you have a consistent frame of reference for all the new stuff?

I am thinking of building a chassis jig table, before I do any work on the rear frame rails, to set ride height, and brace up things before any cutting is done, and make sure everything is level and square so my measurements are good.

Any other ideas? would a chassis jig table be overkill?

sfi427 · 11-20-2009, 12:53 PM

Art Morrison Triangulated 4-bar. Nice set up but I had to do mods to the rear seat. Fit may be better on a fastback.

65Fastback434 · 11-20-2009, 04:13 PM

Wow that looks pretty wicked! Crazy exhaust. Do you have any more pics? perhaps without the suspension installed?

65Fastback434 · 11-20-2009, 04:16 PM

So the new boxed frame rails connect to the original torque boxes, and then is that a piece of the old frame rail at the back that connect the new frame rails to the rear cross member?

sfi427 · 11-23-2009, 11:14 PM

Quote:

Originally Posted by 65Fastback434

So the new boxed frame rails connect to the original torque boxes, and then is that a piece of the old frame rail at the back that connect the new frame rails to the rear cross member?

I tied the ends of the subframe to the factory rail stubs. It made it easy that way so the bumper brackets would still be connected to the new frame. The front is tied to the torque boxes as well as 2X3 subframe connectors. The sleeves for the exhaust are 4" pipe. The exhaust is 3" stainless. It exits out the middle like a stock 69 Shelby but with much less restriction than the stock Shelby cast pod. It was a pain to fabricate but worth it.
The cage is for the fuel cell as the frame rails are 40" wide.